Thermal storage device

ABSTRACT

A refrigeration device ( 10 ) including a generally cylindrical tank ( 11 ) including outer sidewalls ( 14 ) adapted to contain a first fluid. The device further includes a heat exchange fluid inlet ( 17 ) to the tank, a heat exchange fluid outlet ( 18 ) from the tank and a refrigeration unit ( 60 ). At least one hollow refrigeration evaporator coil ( 19, 20 ) is in fluid communication with the refrigeration unit by means of refrigerant feed and extraction pipes ( 61  and  62 ). The coil is helically disposed within the tank for freezing the first fluid adjacent the coil, such that in use the frozen fluid and the sidewalls together define a substantially helical path to direct the flow of a heat exchange fluid from the inlet to the outlet.

FIELD OF THE INVENTION

The present invention relates to thermal storage devices and inparticular to the off-peak provision of refrigeration for subsequentuse. It has been developed primarily for use as a refrigeration deviceand will be described hereinafter with reference to this application.

BACKGROUND

The following discussion of the prior art is intended to present theinvention in an appropriate technical context and allow its significanceto be properly appreciated. Unless clearly indicated to the contrary,however, reference to any prior art in this specification should not beconstrued as an admission that such art is widely known or forms part ofcommon general knowledge in the field.

It is known to freeze water and other suitable liquid media duringperiods of low thermal load and later to draw upon the resultant icebank when the required loading and energy costs may be higher. Thismechanism provides a low-cost means of cooling. In addition, thesubstantially constant temperature of the ice can be utilised to providea constant process temperature, and the ice bank enables the system morereadily to meet peak cooling demands.

One known form of thermal storage device employs a number of spacedgrids of refrigerant pipes extending between inlet and outlet manifolds.These grids are closely packed within a water tank so that over a periodof around eight hours, they freeze most of the water within the tankinto a solid block of ice. When the process is reversed, water is passedthrough the tank around the periphery of the block, which progressivelymelts the ice and chills the water flowing around it.

These known devices suffer from various shortcomings. Because the iceblock is initially one solid mass, only its outer surface is exposed tothe water flow. This flow can also bypass the block by fast tracking tothe tank outlet as the ice melts, thereby reducing the degree of heattransfer and hence the efficiency of the device. The melting can alsooccur in an irregular manner causing further inefficiency.

Attempts have been made to overcome this deficiency by promoting watercirculation through agitation, for example by aeration or by paddles.However, both of these methods increase the complexity and therefore themanufacturing cost of the device, and also reduce its efficiency becauseof the additional energy consumption associated with the agitationmechanism itself.

Another disadvantage of prior art devices is that the spaced grids ofrefrigerant piping cannot easily be changed. Switching pipes may benecessary for repair but also for changing the pipe material to suitdifferent refrigerants. For example, copper is unsuitable for use withammonia.

It is an object of the present invention to overcome or ameliorate oneor more of the disadvantages of the prior art, or at least to provide auseful alternative.

SUMMARY OF THE INVENTION AND OBJECT

According to a first aspect of the invention, there is provided athermal storage device including:

a generally cylindrical tank including outer sidewalls adapted tocontain a first fluid;

a heat exchange fluid inlet to said tank;

a heat exchange fluid outlet from said tank;

a refrigeration unit;

at least one hollow refrigeration evaporator coil in fluid communicationwith said refrigeration unit by means of refrigerant feed and extractionpipes, said coil being helically disposed within said tank for freezingthe first fluid adjacent the coil, such that in use the frozen fluid andthe sidewalls together define a substantially helical path to direct theflow of a heat exchange fluid from said inlet to said outlet; and

a generally cylindrical column removably mounted to the tank andextending coaxially through an interior region of the tank, the columndefining an inner sidewall of the tank.

Preferably, the column supports the coil.

Preferably, the device includes valve means disposed selectively tointroduce relatively hot fluid into the coil to rapidly heat an outersurface thereof, so as to crack and create fissures in the frozen fluidand thereby increase the rate of freezing of the first fluid. Morepreferably, the valve means include a reversing valve operable on therefrigerant feed pipe.

Preferably, the device further includes at least one sensor fordetecting propagation of an interface between a frozen phase andsurrounding liquid phase of the first fluid.

In a preferred embodiment, the heat exchange fluid has a lower freezingpoint than the first fluid.

According to a second aspect of the invention, there is provided amethod of operating a device as defined above, including the steps of:

directing an evaporative fluid through the evaporator coil so as toreduce the temperature of an outer surface of the coil to a temperatureless than or equal to the freezing point of the first fluid,

thereby causing said first fluid to freeze on the outer surface of theevaporator coil,

allowing sufficient time for an interface between solid and liquidphases of the first fluid to advance such that the frozen liquid and thesidewalls together define a substantially helical path; and

directing a heat exchange fluid to flow along said helical path suchthat the temperature of the heat exchange fluid progressively dropstoward the temperature of the frozen first fluid.

Preferably, the method includes the further step of periodicallyinjecting a hot fluid into the coil to crack and create fissures in thefrozen fluid and thereby increase the rate of freezing of the firstfluid.

Preferably, the hot fluid is selectively injected when required at arate and temperature sufficient to fracture discrete blocks from thefrozen fluid, and thereby significantly increase the exposed surfacearea of the frozen fluid.

Preferably also, the method includes the further step of recovering theheat extracted from said heat exchange fluid for use in otherapplications.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described, by way ofexample only, with reference to the accompanying drawings in which:

FIG. 1 is a sectional side elevation of a refrigeration device accordingto the invention;

FIG. 2 is a sectional plan view taken on line 2-2 of FIG. 1;

FIG. 3 is a sectional side elevation of another embodiment of therefrigeration device with a slightly changed evaporator coil dispositionto ease manufacture;

FIG. 4 is a schematic layout of a refrigeration circuit incorporatingthe refrigeration device, the circuit being configured to chill water toice and recover waste heat; and

FIG. 5 is a schematic layout of the device showing various controlcomponents.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIGS. 1 to 3 of the drawings, the refrigeration device 1includes a right circular cylindrical tank 2 having a centralcylindrical column 3 defining an annular operative chamber 4 bounded byouter and inner sidewalls 5 and 6 respectively.

The tank is positioned vertically about an axis 7 and includes a heatexchange fluid inlet 8 adjacent the top of the tank and a correspondingoutlet 9 at the base of the tank. In this embodiment, the heat exchangefluid is water.

The central column 3 supports a pair of refrigeration coils 10 and 11,each concentrically disposed within the tank in the form of a regularhelix. The helices are of identical pitch in terms of revolutions perunit length, but of differing radii and are axially positioned such thatin a section taken on a plane including the tank axis, at any givenpoint the adjacent sections of the respective coils are alwayssubstantially disposed on a common radial line, as shown in FIG. 3.

The coils 10 and 11 are each in fluid communication with a refrigerationunit 13 by means of a respective refrigerant feed pipe 13 and anextraction pipe 14.

In this embodiment, the coils are horizontally spaced in section asillustrated. In alternative embodiments where only a single refrigerantcoil is used or where two or more coils are provided but are nothorizontally aligned, the invention will operate, but may do so lessefficiently.

Agitation means in the form of an axially extending perforate tube 15 ismounted within the tank 2. The tube includes an array of axially spacednozzles 16 for injecting material into the water to increase its flowrate and turbulence. In this embodiment, the nozzles take the form ofapertures in the tube. The increased agitation has the effect ofreducing the effective freezing point of the water, thereby allowing itstemperature to drop to below 0° C. without forming ice. The injectionmay be powered from the supply water or using a separate pump.

A typical refrigeration circuit including a refrigeration device asdescribed above is illustrated in FIG. 4. The circuit includes acompressor 17, suction accumulation vessel 18, oil trap 19, condenser20, thermostatic expansion valve 21 and the evaporator coils 11 whichare immersed in the thermal storage tank 2. The thermostatic expansionvalve is typically controlled by sensing the exit temperature of therefrigerant from the evaporator tube by means of a sensor 22. Thethermostatic expansion valve controls the ice temperature, which istypically set at 10° C.

A typical control system for the device is illustrated in FIG. 5 andincludes several features in common with the refrigeration circuit, withcorresponding reference numerals indicating like features.

The control system includes valve means 23 disposed selectively tointroduce relatively hot fluid into the coil to rapidly heat an outersurface thereof, so as to crack and create fissures in said frozen fluidand thereby increase the rate of freezing of the first fluid. The valvemeans including a reversing valve 23 operable on the refrigerant feedpipe.

An electronic expansion valve 21 is operable on the refrigerant feedpipe for metering the evaporation of refrigerant gas within theevaporation coil. A pair of sensors 24 and 25 are positioned to detectpropagation of an interface between a frozen phase and surroundingliquid phase of the first fluid between respective predetermined maximumand minimum positions in response to progressive freezing of the firstfluid.

The control system further includes a temperature sensor 26 on theoutlet and a temperature sensor 27 on the inlet. The outlet sensor 26ensures that water is supplied at the design temperature. The inletsensor 27 detects the return temperature of the heat exchange fluidentering the tank. A timer 28 is included to activate the refrigerationunit in off-peak periods. A manual override mechanism (not shown) isalso provided to permit activation of the refrigeration unit outside ofoff-peak periods.

An inlet pressure sensor 29 detects pressure in the inlet and an outletpressure sensor 30 detects pressure in the outlet. The inlet and outletpressure sensors, among other functions, detect a build-up of pressureresulting from a blockage in the system. High and low pressure sensors31 and 32 ensure that the refrigeration compressor operates within safeworking limits. A condenser fan controller 33 controls condenser fans(not shown) and optimises condensation.

A solenoid valve 34 is positioned to close the refrigeration circuit andalso to allow multiple thermal storage devices to be connected to asingle refrigeration unit. A pump 35 is provided and is responsive to aspeed controller 36. The speed controller adjusts the pump speed basedon the temperature sensed by the inlet temperature sensor 27. Where thistemperature is higher than the optimal level, the pump speed isincreased accordingly.

Electronic control means 37, including a microprocessor 38, control theoperation of the device in response to adjustable set-points andpredetermined system parameters including outputs from the varioussensors described above.

The controller is adapted for remote monitoring, supervision andadjustment of various system parameters. The remote monitoring may beachieved through a modem and phone line, the Internet, or wirelessdevices such as GSM phones.

It will be appreciated that some or all of the control devices may beused, depending on the particular application. For example, in more costsensitive market segments, or where maximum efficiency is of secondaryimportance, the condenser fan controller 33 and electronic expansionvalve 21 may be omitted.

In use, the tank is filled almost completely with water and therefrigeration coils are operated as an evaporator to freeze the adjacentwater. As this freezing process continues, the growing ice helicesbridge the gap between the refrigeration coils and coalesce into asingle ice helix.

The coils are positioned with respect to the tank sidewalls such thatthe freezing process can be permitted to continue until the ice helixtouches or almost touches one or both of the inner and outer sidewalls.This stage is determined by the sensor 66, which detects the maximumextent of ice growth. The sensor may, for example, detect this stagethrough changes in resistance between the liquid and solid phases.

The over freezing of the ice mass may also be detected by the placementof thermocouples in areas where the ice mass is to stop forming. Oncethe temperature of these thermocouples falls below freezing point, theice mass has progressed to its design limit and the refrigeration plantis shut down.

It has been found that if the thermocouples are repetitively frozen inthe ice mass, they may become damaged and unreliable. This defect hasbeen ameliorated by placing the thermocouples 39 in a metal tube 40which is partially filled with a low freezing point liquid such asethylene glycol to effect heat transfer to the thermocouples, thusenabling the thermocouples to sense temperatures below the freezingpoint of water without being subjected to the stress of being embeddedin a frozen fluid.

The second sensor 25, which detects minimum extent of ice growth,ensures that a sufficient amount of ice is present to provide outputwater at the design temperature.

During the freezing of the ice helix, the ice formed around the helicalevaporator coils increases in thickness and forms an insulation bafflerto the further transfer of heat from the water in the tank to theevaporator coils thus slowing down the rate of ice growth. It has beenfound that if the evaporator coils are suddenly heated, the ice growthon the coils cracks, creating water paths through fissures in the ice tothe evaporator coils and thus improving heat transfer and allowing morerapid growth of the ice layer around the coils.

With sufficient heat, it has also been found that discrete blocks ofcracked ice can be broken away from the ice helices entirely so as tofloat freely in the tank, thereby further increasing the rate of iceformation on the coils, as well as increasing the total effectivesurface area of ice available for heat transfer.

Referring to FIG. 4, the sudden introduction of heat into the evaporatorcoils is accomplished by means of hot gas injection from therefrigeration compressor 17. The injection is controlled by opening thereversing valve 23 on the refrigerant feed pipe 13 for a short period oftime, thereby very rapidly raising the temperatures of the coils.

The hot gas injection may be controlled on a time basis throughout thefreezing cycle or alternatively by means of sensors that detect the rateof ice build up or heat removal from the chilled water. In oneembodiment, the sensor is a monitor on the refrigeration circuit thatmeasures the rate of heat rejection at the condenser.

The heat rejected at the condenser may be recovered in order to heatwater for other uses within the establishment where the thermal storagedevice is installed. This is accomplished by using the heat exchanger 41as a condenser for the refrigeration unit. The water to be heated entersthe heat exchanger through the inlet 42 and leaves through the outlet43.

The flexibility of the refrigeration circuit may be improved by theaddition of three way valves 44 to provide for use of a regular aircooled condenser 20 in the form of a cooling tower. Alternatively, thehot water heat exchanger may be used as a condenser. This may beparticularly advantageous where the temperature of the water beingheated for other uses rises to a level at which the efficiency of therefrigeration unit is adversely affected.

Once the ice helix has been formed, the thermal storage tank is readyfor use as a chiller by passing water through the tank from the inlet 8to the outlet 9. The ice helix, together with the tank sidewalls,defines a substantially helical path for the water as it passes throughthe tank. This path is well defined and free-flowing to reduce thetendency of the water to by-pass the helix adjacent the tank sidewalls.

It is also apparent that the water is in constant heat exchange contactwith the ice over a distance much greater than the length of the tank.The actual contact length is dependent on the size of the tank and thepitch of the coils. The pitch is approximately 12° in this embodimentand on that basis, a tank of 1.9 metres in length produces approximately44 metres of helical path.

Water flow along the helix is promoted by an appropriately directedinlet 8 and outlet 9. Because of the substantially constant temperatureof the ice, provided the water flow is not excessive, chilled water canbe output at a substantially constant depressed temperature of around0.5° C.

The water temperature may be further depressed to below 0° C. by addingethylene glycol to the chilled water supply to depress the freezingtemperature of the chilled water below 0° C.

The tank may be formed of any suitable material such as polyethylene.The central column in the preferred embodiment is formed ofpolyvinylchloride piping and serves to support the refrigerant coil bymeans of a plurality of copper brackets 46.

The tank includes a removable lid 47 to permit easy access to thehelical coils, which can be removed integrally with the central column.This allows the refrigeration coils to be easily removed for inspection,cleaning or replacement. The lid, and supplementary insulation ifrequired, effectively isolates the system from external environmentalinfluences.

In other embodiments, the tank maybe of non-circular cross-section withcorresponding changes to the helix so that the ice can interactefficiently with the adjacent sidewalls. The central column can also beomitted, though with an increased tendency for water by-passing the icehelix. Particularly in this case, additional refrigeration coils may beadded.

The tank may also be operated in a horizontal configuration. This isless desirable because higher flow pressures are required to compensatefor the loss of gravity feed. It is also more difficult to produce acomplete ice helix since the tank must desirably be full of water in itshorizontal configuration and this involves additional means to providefor expansion of the ice on freezing.

It will be appreciated that the thermal storage device described aboveis well adapted for use in many applications requiring chilled water.These include industrial and agricultural processes such as beveragechilling, air-conditioning and food processing. The unit may be used toreplace air-conditioning cooling towers without the need for specialchemicals to control legionella bacteria. For heating applications, thedevice may be operated in a reverse cycle mode. In this mode, thereversing valve 23 is opened and hot gas is injected into the coils fora time sufficient to heat the water in the tank to the desiredtemperature.

In terms of effectiveness, efficiency, size, reliability and overallperformance, the invention provides both practical and commerciallysignificant improvements over the prior art.

Although the invention has been described with reference to specificexamples, it will be appreciated by those skilled in the art that it maybe embodied in many other forms.

1. A refrigeration device including: a generally cylindrical tankincluding outer sidewalls adapted to contain a first fluid; a heatexchange fluid inlet to said tank; a heat exchange fluid outlet fromsaid tank; a refrigeration unit; at least one hollow refrigerationevaporator coil in fluid communication with said refrigeration unit bymeans of refrigerant feed and extraction pipes, said coil beinghelically disposed within said tank for freezing the first fluidadjacent the coil, such that in use the frozen fluid and said sidewallstogether define a substantially helical path to direct the flow of aheat exchange fluid from said inlet to said outlet; and a generallycylindrical column removably mounted to said tank and extendingcoaxially through an interior region of the tank, said column definingan inner sidewall of said tank.
 2. A device according to claim 1,wherein, in use, said column supports said coil.
 3. A device accordingto claim 1 or claim 2, further including valve means disposedselectively to introduce relatively hot fluid into said coil to rapidlyheat an outer surface thereof, so as to crack and create fissures insaid frozen fluid and thereby increase the rate of freezing of the firstfluid.
 4. A device according to claim 3 wherein said hot fluid isinjected at a rate and temperature sufficient to fracture discreteblocks from said frozen fluid, and thereby significantly increase aneffective exposed surface area of the frozen fluid.
 5. A deviceaccording to claim 3 or claim 4 wherein said valve means include areversing valve operable on said refrigerant feed pipe.
 6. A deviceaccording to any one of the preceding claims including an electronicexpansion valve operable on said refrigerant feed pipe for metering theevaporation of refrigerant gas within said evaporation coil.
 7. A deviceaccording to any one of the preceding claims, further including at leastone sensor within said tank for detecting propagation of an interfacebetween a frozen phase and surrounding liquid phase of the first fluid.8. A device according to claim 7 including a pair of said sensors fordetecting propagation of the interface between respective predeterminedmaximum and minimum positions in response to progressive freezing of thefirst fluid.
 9. A device according to any one of the preceding claims,further including a temperature sensor on said outlet.
 10. A deviceaccording to any one of the preceding claims, further including atemperature sensor on said inlet.
 11. A device according to any one ofthe preceding claims, further including a timer adapted to activate saidrefrigeration unit in off-peak periods.
 12. A device according to claim11, further including a manual override mechanism disposed to permitactivation of said refrigeration unit outside of off-peak periods.
 13. Adevice according to any one of the preceding claims, further includingan inlet pressure sensor for detecting pressure in said inlet.
 14. Adevice according to any one of the preceding claims, further includingan outlet pressure sensor for detecting pressure in said outlet.
 15. Adevice according to any one of claims 7 to 14, further includingelectronic control means to control the operation of said device inresponse to predetermined system parameters.
 16. A device according toclaim 15 wherein said electronic control means include a microprocessorresponsive to predetermined system parameters including outputs fromsaid sensors.
 17. A device according to any one of the preceding claims,further including agitation means disposed within the tank to agitateand thereby reduce the effective freezing point of said heat exchangefluid.
 18. A device according to claim 17 wherein said agitation meansincludes at least one nozzle within said tank for injecting materialinto the heat exchange fluid, thereby increasing its flow rate andturbulence.
 19. A device according to claim 18, wherein said agitationmeans include an axially extending perforate tube mounted within thetank, said tube including an array of said nozzles in the form ofapertures or perforations axially spaced along its length.
 20. A deviceaccording to any one of the preceding claims, wherein said coil isconfigured substantially in the shape of a regular helix.
 21. A deviceaccording to claim 20, wherein the pitch of said coil is between 5° andaround 20°.
 22. A device according to claim 20 or claim 21, wherein thepitch of said coil is between 10° and around 15°.
 23. A device accordingto any one of claims 20 to 22, wherein the pitch of said coil isapproximately 12°.
 24. A device according to any one of the precedingclaims, including a pair of said coils disposed generally concentricallywithin said tank.
 25. A device according to claim 23 including three ormore of said coils disposed generally concentrically within said tank.26. A device according to claim 24 or claim 25 wherein the pitch of saidcoils is substantially identical.
 27. A device according to any one ofthe preceding claims wherein said tank is generally right cylindrical,and generally circular in cross-sectional profile.
 28. A deviceaccording to any one of the preceding claims wherein said heat exchangefluid has a lower freezing point than said first fluid.
 29. A deviceaccording to any one of the preceding claims wherein said heat exchangefluid includes ethylene glycol.
 30. A method of operating a device asdefined in any one of the preceding claims, including the steps of:directing an evaporative fluid through the evaporator coil so as toreduce the temperature of an outer surface of the coil to a temperatureless than or equal to the freezing point of the first fluid, therebycausing said first fluid to freeze on the outer surface of theevaporator coil, allowing sufficient time for an interface between solidand liquid phases of the first fluid to advance such that the frozenliquid and the sidewalls together define a substantially helical path;and directing a heat exchange fluid to flow along said helical path suchthat the temperature of the heat exchange fluid progressively dropstoward the temperature of the frozen first fluid.
 31. A method accordingto claim 30 including the further step of periodically injecting a hotfluid into said coil to crack and create fissures in said frozen fluidand thereby increase the rate of freezing of the first fluid.
 32. Amethod according to claim 31 wherein said hot fluid is injected at arate and temperature sufficient to fracture discrete blocks from saidfrozen fluid, and thereby significantly increase the exposed surfacearea of the frozen fluid.
 33. A method according to any one of claims 30to 32 including the further step of injecting material into the heatexchange fluid to increase its flow rate or turbulence.
 34. A methodaccording to any one of claims 30 to 33 including the further step ofrecovering the heat extracted from said heat exchange fluid for use inother applications.
 35. A method according to any one of claims 30 to 34wherein said heat exchange fluid has a lower freezing point than saidfirst fluid.
 36. A method according to any one of claims 30 to 35wherein said heat exchange fluid includes ethylene glycol.